In recent years, large attention has been given to a hybrid vehicle equipped, as drive power sources, with a battery, inverters and motors driven by the inverters in addition to a conventional engine.
Japanese Patent Laying-Open No. 10-191503 has disclosed a hybrid vehicle that can perform limp-home run (batteryless run) in which a power generator is used for driving a vehicle while avoiding use of a battery that has a certain failure. In this hybrid vehicle, when it is determined that the battery is unusable, a system main relay is turned off to isolate a battery from a power generator and a load, and the power generator operates so that a power generation output of the power generator follows the load.
In this hybrid vehicle, even when the battery that can function as an electric power buffer is isolated, the power generation output of the power generator follows the load so that a smoothing capacitor can be protected from overvoltage breakdown.
However, when the battery is suddenly isolated due to a break in a wire of the system main relay, or when a booster device malfunctions in a system having the booster device between the battery and the inverter, an unexpected overvoltage may rapidly occur due to the power supplied from the power generator. In view of this situation, therefore, even the foregoing hybrid vehicle must keep a capacitance margin of the smoothing capacitor to a certain extent, and this increases sizes of the capacitor.